The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The temperature-sensitive and/or time-sensitive indicators are known from the prior art which can be arranged, for example, on a label, on product packaging or directly on a product. Such indicators are suitable for indicating whether a product associated with the indicator was stored in a sufficiently cool manner during the time period it was provided with the indicator and/or whether the product has passed a predetermined age. Indicators are available which are only time-sensitive and are thus independent of the respective temperature so that they can only mark the age of a product. Furthermore, time-sensitive and temperature-sensitive indicators are known which change both in dependence on the respective time past and in dependence on the temperature they have respectively been exposed to. Finally, indicators are also known which are only temperature-sensitive. The present invention relates to all named types of indicators.
A substrate is known from German patent DE 198 03 208 C2 for application onto age-sensitive and temperature-sensitive products or their packaging having a time/temperature indicator arranged in the region of the substrate. The starting time of this time/temperature indicator can be set as desired by light radiation, in particular by UV light radiation, with the indicator containing a matrix and at least one reversible, rechargeable crystalline substance which is embedded therein and which has photochromic properties based on transfer reactions.
The indicator can change color in a photo-induced manner due to the photochromic properties, with a time-dependent and temperature-depending fading taking place subsequently to the color change.
It is known to apply an irreversible photosensitive indicator serving as proof against forgery as a film to the time/temperature indicator. The material of the irreversible indicator turns red, for example, when it is exposed to UV light.